1. Field of the Invention
The present invention relates to a tuning circuit capable of increasing Q thereof up to a desired predetermined value wherein a negative resistance circuit is added to a resonance circuit comprising an inductor and an capacitor.
2. Description of the Related Art
There are methods using a negative resistance circuit as a method for increasing Q of a tuning circuit. As one of them a regenerative detection method has been many used. However, it is necessary to set up a positive feedback circuit whenever a tuning frequency is changed because the positive feedback circuit is used on the eve of oscillation.
So, the inventor has filed an automatic setting method of a tuning frequency in order to improve this drawback, which is disclosed in Japanese Patent Application No. 2000-400944. Said method is as follows. In a tuning circuit comprising a resonance circuit consisting of an inductor L and a capacitor C and a negative resistance circuit connected to it in series, when a composite value (an effective resistance value) of a negative resistance value of the negative resistance circuit and an original series resistance value of the tuning circuit becomes negative, the tuning circuit oscillates, when the effective resistance value is caused to oscillate in this condition, an amplitude of an oscillation signal varies. So, the original series resistance value of the tuning circuit can be calculated from relation between two values of the negative resistance value by detecting the amplitudes of the oscillation signal corresponding to said two values. Since xcfx89L is known, a negative resistance value to be set can be obtained by determining a value of Q to be set if this original series resistance value is known.
Since content of the above operation is described in the above application, detailed explanation thereof is omitted but a several operations containing multiplication and division are necessary to do it. Therefore, this method requires operational circuits and hold circuit for holding operational result therefrom.
In the above method, requirement of the hold circuit of operational result is inevitable but it is desirable to get along all right without complicated operational circuits (ex. multiplication and division circuits) if possible.
An object of the invention is to provide a tuning circuit capable of setting Q thereof to a desired high predetermined value by a very simple circuit without using the above operational circuits.
In order to achieve the above object a tuning circuit of the invention includes a resonance circuit composed of an inductor and a capacitor and a negative resistance circuit connected to said resonance circuit in series and is characterized by employment of a negative resistance control means for setting a negative resistance value of said negative resistance circuit so as to make an effective resistance value of said tuning circuit be negative to oscillate it and for varying said negative resistance value so as to make it be positive; and stop and hold means for stopping variation of said negative resistance value at a time when said effective resistance value becomes a positive predetermined value and for holding said value.
In the present invention, it is preferred that said negative resistance control means includes a counter for counting a clock signal and is constituted to control said negative resistance value in response to a count output of the counter, and said stop and hold means a latch circuit for stopping count of the clock signal due to said counter said time and for holding the count output at said time. Further, in the present invention, it is preferred that said stop and hold means comprises setting means for setting said positive predetermined value and that the tuning circuit may comprise means for switching input and output terminal of said tuning circuit therefrom.